This invention relates to electrophotographic cadmium sulfide material, and more particularly relates to a copper activated cadmium sulfide material for use in the xerographic mode and to photoreceptor devices incorporating such material.
Cadmium sulfide activated with copper or silver is a well-known photoconductive material, which is useful, for example, in photocells, detectors and electrophotography.
In electrophotography, a photoreceptor surface bearing a photoconductive materal is given an electrical charge in the dark, and then an image is formed by the selective discharge of certain areas by illumination corresponding, for example, to the light reflective areas of an original document. The thusformed latent image is then transferred to copy paper and developed by the adherence of toner particles to the charged areas of the copper.
The initial charge on the photoreceptor surface may either be positive (as in the so-called canographic mode) or negative (as in the so-called xerographic mode).
While cadmium sulfide photoconductive material can be used in a photoreceptor device to obtain a surface charge of either polarity, its major use up to the present time has been mainly in the canographic mode. See, for example, U.S. Pat. No. 4,239,844 assiged to the present assignee, describing copper-activated cadmium sulfide containing cadmium selenide addition for use in the canographic mode.
Among the characteristics needed for a photoreceptor device for use in the xerographic mode are high charge acceptance, low residual voltage, high contrast voltage and high cyclic stability. Cyclic stability is the ability to maintain other electrical properties during multiple charge-discharge copy cycles. Cyclic fatigue is the tendency to gradually lose other electrical properties upon cycling, and is evidenced by a gradually decreasing quality of copies produced by the reprographic equipment.
Because these characteristics are different from those required in most photoconductive applications, the materials used in the photoreceptor devices described herein are referred to as electrophotographic materials, rather than photoconductive materials.
Selenium has been used extensively as the electrophotographic material in the xerographic mode. The development of a cadmium sulfide-type photoconductive material for use in the xerographic mode is attractive, however, due to its expected lower cost relative to selenium. Zinc oxide, another potential low-cost candidate for replacement of selenium, is less desirable than cadmium sulfide because of its lower photosensitivity and poor cyclic stability.